1. Field of the Invention
The invention relates in general to methods of performing thermodynamic cycles for generating mechanical and electrical power, heating, or cooling.
2. Description of the Related Art
The prior art includes combustors and combustion systems that use diluents to cool combustion with relatively poor control over the peak temperature and little spatial control over the transverse temperature profile. Cooling of the combustion products has commonly been done with excess air. Pressurizing this excess air along commonly consumes 40% to 70% (from large turbines to microturbines) of the gross turbine power recovered resulting in low net specific power. I.e., the gross power less compressor and pump power produced per mass flow through the compressor(s) or turbine(s).
Relevant applications of diluent have particularly focused on controlling emissions, flame stability and flame quenching, especially when using ultra-lean mixtures and operating near the combustion limit. (E.g., see Lefebvre, 1998, section 5-7-3; Bathie, 1996, p 139; Boyce, 2002, p 62; Lundquist, 2002, p 85). Some relevant efforts to reduce the use of excess air as diluent have used forms of thermal diluent, such as steam and water, that can remove more heat with less compression work (E.g., U.S. Pat. No. 3,651,641, U.S. Pat. No. 5,627,719, U.S. Pat. No. 5,743,080, and U.S. Pat. No. 6,289,666 to Ginter, U.S. Pat. No. 5,271,215 to Guillet, U.S. Pat. No. 6,370,862 to Cheng).
Temperature control throughout the thermodynamic cycle is important for efficient operation. Controlling the peak temperature and profile of the energetic fluid delivered to an expander results in better efficiencies of the cycle (Gravonvski, 2001). It is difficult to control the temperature profile using excess air or steam as commonly used to cool cycle components and special mixing devices. The irregularities in the spatial and temporal temperature distribution of flows require greater design margins than preferred to compensate for the uncertainties in the temperature profiles. (E.g., Malecki et al, “Application of and Advanced CFD-Based Analysis System to the PW600 Combustor to Optimize Exit Temperature Distribution—Part 1”, 2001). The problem of temperature irregularities is heightened by changes in work load.
Various thermodynamic cycles have been proposed to improve heat recovery and system efficiency. The conventional Combined Cycle (CC) utilizes a Heat Recovery Steam Generator (HRSG) to generate steam which is expanded through a second (steam) turbine. This results in high capital costs. Consequently combined cycles are mostly used in base load applications. In the Steam Injected Gas Turbine (STIG) cycle, steam is generated in a similar HRSG and is injected upstream of the expander. This uses the same gas turbine with a higher mass flow. By only being able to deliver steam, the STIG cycle is limited in its ability to recover lower temperature heat. High water treatment costs and water availability are often stated as a major objections to more widespread use of the STIG. The CHENG cycle is similar to the STIG cycle. Similar objections have been raised for it as the STIG cycle.
The Recuperated Water Injection (RWI) cycle utilizes a recuperator to recover heat from expanded fluid into incoming compressed. It uses water injection on the intake of the recuperator to improve heat recovery. This is similarly limited by the air saturation limit. The Humidified Air Turbine (HAT) cycle humidified intake air through a saturator. The Evaporated Gas Turbine (EvGT) is a similar cycle. While utilizing lower quality water, the HAT and EvGT cycles are limited in the amount of deliverable diluent by air saturation limits. An EvGT cycle has been demonstrated at LUND University in Sweden. Otherwise these RWI, HAT, and EvGT cycles have been little used, possibly because of relatively high capital costs. The HAWIT cycle has been proposed to reduce capital costs. It utilizes direct contact heat exchangers to reduce the cost of surface heat exchangers used in the HAT cycle. It has lower costs but lower efficiency than the HAT cycle. The relative efficiency and internal rate of return for these cycles were compared by Traverso (2000).
Conventional heat recovery methods have particular difficulty in recovering useful heat below the temperature of steam formed from the expanded fluid with sufficient pressure to reinject upstream of an expander, or within a steam expander. Much heat energy continues to be lost as the expanded is exhausted. Conventional methods of recovering heat from the expanded fluid (after the hot energetic fluid has been expanded to extract mechanical energy) often seek to use high temperature recuperators to heat the large volume of excess cooling air. E.g. air to air recuperators approaching 700° C. These result in high cost and expensive maintenance, where the recuperator alone may exceed 30% of system costs and 80% of the maintenance in micro-turbines.
Using diluents other than air have resulted in further expenses in diluent supply and recovery in relevant cycles for power generation such as “wet” cycles like STIG and HAT. Relevant cycles with typical heat and diluent recovery systems typically need to add “make-up” diluent to compensate for inefficiencies of the system and to reduce operation costs.
Thermodynamic cycles that use a diluent beyond the oxidant containing fluid often need to recover that thermal diluent for pollution and/or economic reasons. (For example, Italian Patent TO92A000603 to Poggio and Agren, “Advanced Gas Turbine Cycles with Water-Air Mixtures as Working Fluid”, 2000). Such processes have been expensive. Make-up diluent is commonly needed because of inefficiencies in the recovery process (Blanco, 2002).
In the addition of a thermal diluent, fluid filtering and cleanup has been required to prepare the diluent to be delivered to the thermodynamic cycle system (Agren, “Advanced Gas Turbine Cycles with Water-Air Mixtures as Working Fluid”, 2000; SPE “Mashproekt”, “Aquarius Cycle”, http://www.mashproekt.nikolaev.ua). Such conventional methods add substantial expenses.
Pollutants are becoming a common concern throughout the world and their control is becoming more important. Relevant art methods of adding water often exacerbate formation of some pollutants such as while decreasing others. (e.g. See Lefebvre, 1998, p 337 on CO vs NOx). Control of pollutants to stringent legislated methods has often required additional components at substantial further capital and maintenance costs. Many of these pollutant control devices have short lives compared to the overall plant life resulting in further maintenance expenses. Major firms appear to have made a concerted effort to shift to dry excess air to achieve low NOx emissions and to avoid the use of steam as diluent.
Lefebvre (1998, p 337) demonstrates that conventional wisdom discourages water injection into turbine power systems. The cost of providing and treating water is frequently noted as a substantial hindrance. Commentators expect efficiencies to drop as more water or steam is added to the cycle (Pavri and Moore 2001, p 18).
Thermodynamic cycles are sometimes used for both mechanical work and heating. The heat produced by the combustion process may be used for heat in assorted applications from steam production to district heating. These “combined heat and power” (CHP) applications have been limited by the design of the CHP device. If the demand for heat or power deviates from the design of the CHP system, the efficiencies are greatly reduced, especially when providing hot water.